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From: Matthew Leach <matthew.leach@arm.com>
To: Paul Turner <pjt@google.com>
Cc: linux-kernel@vger.kernel.org, Dietmar Eggemann <Dietmar.Eggemann@arm.com>,
        Peter Ziljstra <peterz@infradead.org>
Subject: A question regarding TG load tracking functions in fair.c
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Date: Fri, 28 Feb 2014 12:04:19 +0000
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Hi Paul,

I have a question regarding two lines of code in the functions
__update_cfs_rq_tg_load_contrib and __update_tg_runnable_avg. AFAICS
these functions update a load-tracking signal for the local RQ and an
aggregated signal in the TG for all RQs that belong to this TG. The
update is surrounded by conditions:

  if (abs(contrib) > cfs_rq->tg_runnable_contrib / 64) {

for __update_tg_runnable_avg and

  if (force_update || abs(tg_contrib) > cfs_rq->tg_load_contrib / 8) {

for __update_cfs_rq_tg_load_contrib. Is this an optimisation to prevent
the signals being updated for small variations? Also, could you please
shed some light on the specific numbers used?

Thanks,
-- 
Matt